Vacuum drying method and apparatus

ABSTRACT

Method and apparatus for drying granular resin material by drawing vacuum over heating resin material in a vessel, while periodically purging the vessel with the material therein with dry air and bathing the vacuum dried material with dry air until furnished to a processing machine.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is filed under 35 USC 120 as acontinuation-in-part of co-pending U.S. patent application Ser. No.14/272,721 entitled “Simultaneous Resin Drying and Molding” filed 8 May2014 in the name of Stephen B. Maguire, which in turn was a division andclaimed the benefit of the priority of co-pending U.S. patentapplication Ser. No. 11/402,492 entitled “Resin Drying Method andApparatus” filed 11 Apr. 2006 in the name of Stephen B. Maguire, whichis now U.S. Pat. No. 8,776,392.

This patent application is further filed as a 35 USC 120continuation-in-part of co-pending U.S. patent application Ser. No.14/693,951 entitled “Method and Apparatus for Vacuum Drying GranularResin Material” filed 23 Apr. 2015 in the names of Stephen B. Maguireand Michael Gera.

The benefit of the priority of the '721, '492, and '951 applications isclaimed for this patent application under 35 USC 120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to drying granular or powdery material,preferably granular resin material, prior to processing thereof intointermediate or finished plastic products, preferably by extrusion ormolding.

2. Description of the Prior Art

Plastic resins are initially granular materials and are produced inpellets. These pellets are processed by extrusion or other means inwhich the granular resin pellets are heated until the pellets melt andare then molded or extruded into a desired shape. Typically granularresins melt at elevated temperatures, for example from 300-400° F.,which is well above the boiling point of water.

Many granular resins have affinity for moisture. These hydroscopicresins absorb moisture and cannot be properly processed by molding orextrusion until dried. If processed before they are dry, moisture in theresin boils at or approaching the high plastic molding or extrusionprocess temperature, leaving bubbles and perhaps other imperfections inthe finished plastic product. Hence, hydroscopic granular resins must bedried prior to molding or extrusion.

Some granular resin materials are extremely hydroscopic and becomeunprocessable by molding or extrusion in ten minutes or less afterexiting a dryer, due to the rapid absorption of moisture by the granularresin material.

It is known to dry granular resin material by placing the granular resinmaterial pellets on large shallow trays to a depth of one or two inches,and putting those trays into ovens for several hours. With this approachto granular resin material drying, drying temperatures of up to 150-180°F. but no higher can be used since many granular resin materials beginto soften at 200-210° F.

During the drying process, the granular resin material cannot bepermitted to soften, since it becomes unmanageable. Once granular resinmaterial begins to soften, at temperatures above the boiling point ofwater, the granular resin material pellets stick together in lumps oreven melt into useless masses of solid plastic, making it impossible tofurther process the resin material into a useful article.

U.S. Pat. No. 6,154,980 represents a substantial improvement in dryertechnology providing vacuum-based methods and vacuum drying apparatuswhich substantially accelerate the drying process, providing greaterthroughput of dried granular resin material at lower cost than knownheretofore.

SUMMARY OF THE INVENTION

In one of its aspects, this invention provides a method for continuouslysupplying dried granular resin material for processing from a supply ofmaterial which is excessively moist where the method preferably includessubstantially simultaneously performing the steps of (i) heating aportion of the moist granular resin material to a selected temperatureat which moisture evaporates from the granular resin material when thematerial is exposed to a preselected level of vacuum; (ii) drawing thepreselected level of vacuum over a second portion of the granular resinmaterial which has been heated to the selected temperature for timesufficient to cause the moisture to evaporate therefrom and result inthe second portion of granular resin material being at the preselecteddryness while optionally supplying at least one short burst of heateddry air, preferably at the conclusion of the drying cycle, to the dryingmaterial to achieve an even greater degree of dryness; and (iii)supplying to granular resin material processing equipment from aninventory position granular resin material which was dried to thepreselected dryness by evaporation in the preselected level of vacuumafter having been heated to the selected temperature, and sequentiallyand repeatedly replacing each portion by the next succeeding portion.

In another of its aspects this invention provides apparatus for dryinggranular or powdery material prior to molding or extrusion where theapparatus includes a first material processing chamber, a secondmaterial processing chamber, manifold means for furnishing material tobe dried selectably to one of said first and second processing chamberspreferably most recently having had dried material evacuated therefrom,means for heating material in a selected one of said first and secondprocessing chambers into which material needing to be dried has beenintroduced, means for drawing vacuum over material in a selected one ofsaid first and second processing chambers preferably having had saidmaterial most recently heated therein and means for withdrawing materialfrom said chamber preferably having most recently dried materialtherein.

In yet another of its aspects this invention provides a method fordrying granular powdery material prior to molding or extrusion where themethod includes feeding a first portion of material to a firstprocessing chamber, heating the material in the first processing chamberto a preselected temperature, drawing vacuum over the material in thefirst processing chamber, feeding a second portion of material to asecond processing chamber, heating the material in the second processingchamber while the first portion of material has vacuum drawn thereover,withdrawing material from the first processing chamber when needed formolding or extrusion, and drawing vacuum over the heated material in thesecond processing chamber thereby to evacuate moisture from saidmaterial and prepare such material for molding or extension.

In yet another of its aspects this invention provides a method forsupply of dried granular resin material for processing from a supply ofmaterial which is excessively moist where the method includes heating aportion of the moist material to a temperature at which moistureevaporates at a preselected level of vacuum, drawing at least thepreselected level of vacuum over a second portion of the material whichhas been heated to a temperature and for a time sufficient to cause themoisture to evaporate therefrom and result in the second portion ofmaterial reaching a preselected dryness while periodically introducinghot air into the second portion of material under the preselected vacuumto purge moist air from around such material, and supplying to granularmaterial processing equipment for molding or extrusion a third portionof the material which has been dried to the preselected dryness bymoisture evaporation in the preselected level of vacuum after havingbeen heated.

This invention uses gravity to move granular plastic resin material in avacuum dryer. The granular plastic resin material preferably is heatedin a top heating hopper. The granular plastic resin material is thenpreferably dropped into a vacuum chamber. From the vacuum chamber thegranular plastic resin material is preferably dropped into a retentionhopper.

A plastic product manufacturing process, either molding or extrusion,can preferably draw dry granular plastic resin material from theretention chamber as required, while the heating hopper and the vacuumchamber preferably continuously prepare subsequent batches of granularplastic resin material. The preferable essentially straight downprocessing and drying of granular plastic resin material results in amuch lower cost dry granular plastic resin material as compared togranular plastic resin material dried using known vacuum and other typesof dryers.

In the invention, preferably at least one slide gate selectably allowsand blocks granular plastic resin material downward flow from one partof the dryer to another. Costs are reduced by about forty percent anddrying capacity is actually higher in the advantageously small footprintdryer embodying this invention. The small footprint afforded by thevertical, “stacked” configuration of the dryer is advantageous in thatspace in a plastic manufacturing processing plant, whether an extrusionoperation or a molding operation, is often at a premium.

The vacuum chamber of the granular plastic resin material dryer ispreferably closed by at least one slide gate having a vacuum tight seal.The slide gate preferably closes and seals against an o-ring to providea vacuum tight seal. Use of the slide gate avoids vacuum leakage thatcould occur from contamination that is present everywhere in a plasticmolding or extrusion facility. With the slide gate, plastic dust,flakes, and pellets of granular plastic resin material do not interferewith the vacuum tight seal.

In one of its aspects, the invention introduces dry air into the vacuumchamber periodically. As moisture is released from the granular plasticresin material while under vacuum, a vacuum pump or other source ofvacuum preferably continues to pull the resulting air-water vapormixture from the vacuum chamber. Over several minutes, this mixturechanges to become a very high percentage of water vapor relative to theair remaining in the chamber.

If the moisture in the form of water vapor is not purged from the vacuumchamber, when vacuum is released from the vacuum chamber the resulting“thin” but moisture-laden air would reenter the pellets of granularplastic resin material resident in the chamber and would reverse thedrying that has occurred. To prevent this, in one of its aspects theinvention preferably purges the vacuum chamber of moisture several timeswhile vacuum is present. The invention preferably permits very dry purgeair to enter the vacuum chamber and then draws the resulting mix of thevery dry air and the water vapor-laden air carrying the moisture thathas been drawn out of the resin pellets, out of the chamber.

When drying polyethyleneterephthalate (“PET”), which is usedconventionally for beverage bottles, it is essential that moist ambientair not enter the vacuum chamber at the end of a vacuum cycle. The dryair purge allows effective drying of PET pellets.

To supply such dry purge air, in one of its aspects the inventionpreferably uses a separate dry air source. Suitable dry air can beobtained in several ways. Desirably in the practice of the invention ina preferred manner, the invention utilizes compressed air, which passesthrough at least one oil separator coalescing filter and a compressedair membrane dryer so that the air exiting the oil separator coalescingfilter and the compressed air membrane dryer is extremely dry. This dryair is desirably heated to a desired level for introduction into thevacuum chamber. Since only a relatively small amount of dry air isrequired for purging the vacuum chamber, the compressed air membranedryer can be very small and of very low capacity.

In one aspect of the invention, the hopper in which the granular plasticresin material is initially heated is preferably designed such that hotair enters the bottom of the hopper, passes upwardly through thegranular plastic resin material resident in the hopper, and exits thehopper at the top. As the hot air is passing through the heating hopper,granular plastic resin material may be dropped from the bottom of thehopper into the vacuum chamber, while new granular plastic resinmaterial is added at the top of the hopper. In one aspect of theinvention, the heating hopper preferably holds sufficient granularplastic resin material to provide from three to five hours of residencetime for the granular plastic resin material before exiting the bottomof the heating hopper. In this way, the granular plastic resin materialis exposed to hot, dry air for from three to five hours, which is thetime required for the granular plastic resin material to flow downwardlythrough the heating hopper.

The invention in a preferred manifestation does not dry the granularplastic resin material using “hot” air in the conventional sense. Hotair is used only to bring the granular plastic resin material up to adesired temperature. By carefully controlling the speed of a blower thatmoves the hot air, air flow is preferably adjusted so that the inventionprovides hot air preferably at the correct rate to heat the granularplastic resin material. Viewed differently, most of the useful heat, interms of calories or BTUs, is removed from the hot or “heating” airbefore the heating air arrives at the upper surface of the granularplastic resin material in the heating chamber and is preferably allowedto escape.

In the invention, since the invention is not concerned with heating thegranular resin material during the drying stage, the drying stage,namely the stage during which the pellets are exposed to vacuum in thevacuum chamber, is as short as possible, and may be as little as fifteenor twenty minutes, as contrasted to three to five hours of drying timerequired when using a conventional desiccant dryer.

Preferably, there is no air filter for the heating air in the invention.The heating air is preferably used only once and is preferably vented tothe atmosphere after it has been used for heating and has given up mostof its heat. Most preferably, the hearing air is not recirculated.

The single pass flow of heating air and the elimination of the need fora filter for the heating air is believed unique to this invention.Earlier vacuum dryer designs involved recirculation of air withfiltering being required. In its most preferred manifestation, thisinvention eliminates the need for a filter by having the “heating” airpass through the granular plastic resin material only once. Theinvention further regulates the speed of the blower forcing the airthrough the material to avoid, to the extent possible, loss of unused,residual heat remaining in the “heating” air leaving the heating hopper.Blower speed is preferably adjusted so that only enough heated air, at adesired temperature for the resin material prior to drying, is fed tothe heating hopper at the bottom so that the bottom potion of resin inthe heating hopper reaches the desired final temperature to meet theappetite of the process machine, namely a molding machine or extruder,for dry granular plastic resin material to be molded or extruded.

In one of its aspects, this invention provides a method for dryinggranular resin material, prior to processing of the granular resinmaterial by molding or extrusion, which includes heating granular resinmaterial in a heating hopper, monitoring air temperature at the top ofthe heating hopper, and regulating introduction of heat to the hopperbottom based on monitored air temperature at the top of the heatinghopper.

The method may further proceed by releasing heated granular resinmaterial from the heating hopper for flow downwardly into a vacuumchamber while replenishing the heating hopper from above with freshresin material, preferably in an amount substantially equal to thatreleased into the vacuum chamber. The method preferably proceeds bydrawing vacuum in the vacuum chamber, periodically purging the vacuumchamber interior with dry air while the chamber is under vacuum,draining resin material from the vacuum chamber into a retention hopper,and blanketing dried resin material in the retention hopper with dry airfor so long as the material is resident therein.

Heating the granular resin material preferably further includesintroducing dry heating air into the heating hopper at the heatinghopper bottom.

In still another aspect of the invention, there is provided an improvedmethod for drying granular resin material prior to processing thereof bymolding or extrusion by loading granular resin material into a heatinghopper from above the hopper, introducing heated air into the hopper atthe hopper bottom, monitoring the temperature of the air leaving thehopper at a position above the resin material, and regulating the rateof heated air introduction into the hopper so that monitored temperatureof air leaving the hopper does not exceed a preselected level.

In still another one of its aspects, this invention provides apparatusfor drying granular resin material prior to molding or extrusion of thematerial. Desirably the apparatus includes a heating hopper, a vacuumchamber positioned below the heating hopper, and a retention hopperpositioned below the vacuum chamber. A blower is provided for providingheating air as an air blanket for dried resin material in the retentionhopper.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a dryer in accordance with the invention.

FIG. 2 is a schematic view of a second dryer in accordance with theinvention.

FIG. 3 is a schematic view of a third dryer in accordance with theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND BEST MODE KNOWN FORPRACTICING THE INVENTION

The dryers of the invention require less floor space than a conventionaldesiccant dryer having the same capacity. Additionally, there is nodesiccant maintenance as there is in a conventional desiccant dryerthereby eliminating lost production time, which maintenance is requiredwith a conventional desiccant dryer. Moreover, as desiccant materialdeteriorates, the quality of the plastic granular material being driedsuffers. However, with the dryer of the invention in any of itsembodiments, since there is no desiccant material involved there is norisk of deterioration of product quality from the desiccant material.Performance of dryers according to the invention remains constant anddoes not deteriorate over time.

Dryers according to the invention shorten drying time relative todesiccant dryers thereby avoiding prolonged exposure of the granularresin material to heat. This helps to maintain desired physicalproperties of the resin material.

Dryers according to the invention reduce labor costs in that clean-outtime for hoppers for a color or material change is minimal. Typically, adryer according to the invention requires less than ten minutes of totaltime to clean whereas a conventional desiccant dryer can take up to onehour for cleaning.

Desiccant dryers typically require material feed hoppers to be at leasthalf full for proper air flow. Hence if material usage is low forparticular molding operation, extended exposure to heat in aconventional desiccant dryer may degrade the plastic resin moldingmaterial. There is no such requirement for a full canister for thedryers in accordance with the invention to operate properly.

Test data reveals that operating costs of dryers according to theinvention are less than one-half that of a desiccant dryer having thesame capacity. In many cases operating cost is reduced by as much as 80%over that of a desiccant dryer having the same capacity. Additionally,startup time using a dryer in accordance with the invention is under onehour whereas typical desiccant dryers require four hours or more forstartup.

Use of dryers in accordance with the invention permits materialchangeover time to be reduced to zero if the operator plans about onehour ahead. Color changes in material can be made “on the fly” with nolost time simply by changing the material. When dryers in accordancewith the invention operate on a 20-minute cycle, unused inventory ofblended material represents at most 40 minutes of dryer time, not fourhours as is the case with a conventional desiccant dryer.

Dryers in accordance with the invention minimize the need to exposematerial to be dried to high heat for extended periods, dramaticallyeliminating or minimizing the loss of important physical propertieswhich some materials experience when exposed to high heat for extendedperiods.

Dryers in accordance with the invention permit drying of plastics atlower temperatures than known heretofore; PET heretofore has had to bedried at about 350° F. (180° C.) but with dryers in accordance with theinvention PET can be dried at 245° F. (120° C.)

No cooling water is required for the dryers in accordance with theinvention.

The lower temperature at which the dryers in accordance with theinvention operate allows addition of color concentrates to resinmaterials prior to drying rather than afterwards.

Another problem addressed by the dryers in accordance with the inventionis the separation of preblended materials during extended residencetimes in large hoppers. The chambers of dryers in accordance with theinvention are typically lower in volume than conventional desiccantdryers and may fill and may empty in distinct and complete batchesthereby eliminating the problem of material separation.

The dryers in accordance with the invention require less floor spacethan a desiccant dryer having similar capacity.

The dryers of the invention do not require and do not utilize a dewpoint meter or a dew point control, both of which are subject toreliability problems but are necessary with desiccant dryers.

Dryers in accordance with the invention preferably operate using an airsupply from 75 to 80 psi. This air, which is typically from the airsupply in the facility in which the dryer is used, serves to generatethe required vacuum as well as operate all of the air cylinders of thedryer. To conserve usage of facility air, the venturi vacuum generatorin a dryer of the invention is desirably cycled on and off duringoperation to maintain a minimum vacuum of 25 inches. For largeinstallations, a vacuum pump may be substituted for the venturi vacuumgenerator.

The microprocessor controllers of the dryers preferably includethumbwheel switches or functionally equivalent structure, which are usedto set temperature to which the resin or other granular material is tobe heated prior to drying. Another thumbwheel switch or functionallyequivalent structure is preferably used to set the minimum acceptabletime for a heating cycle and a drying cycle. Typically 20 minutes is thecycle time for acrylic, ABS and polycarbonate while 40 minutes is thecycle time for PET.

FIG. 1 schematically depicts a vacuum dryer embodying aspects of theinvention where the vacuum dryer is designated generally 200. A materialsupply container 202 or equivalent structure is provided as indicatedschematically at the top of FIG. 1; material supply container 202 neednot be a part of vacuum dryer 200.

A preferably tubular material feed line 224 or equivalent structureleads out of material supply 202, preferably downwardly, and connects toa material flow control valve or equivalent structure depictedschematically as 204 in FIG. 1.

Material flow control valve 204 provides material to either of twomaterial feed lines 226, 226A, or equivalent structure, which lead torespective ones of first and second material processing chambers 210,212 or equivalent structure, both of which are illustrated as verticallyoriented cylindrical processing chambers in FIG. 1. Other geometricconfigurations and shapes may also be used.

First and second material processing chambers 210, 212 are equipped withmeans for heating granular material, such as plastic resin, deliveredthereinto via material feed lines 226, 226A. The heating means may beone or more electrical resistance heaters as illustrated schematicallyand designed 214, 216 in first and second material processing chambers210, 212. Alternately and preferably hot air is blown through first andsecond material processing chambers 210, 212 to effectuate heating ofmaterial contained therewithin.

Vacuum dryer 200 further includes a vacuum pump or equivalent structuredesignated generally 208. Vacuum pump 208 draws vacuum within a selectedone of first and second material processing chambers 210, 212 accordingto the position of vacuum control valve 206 or equivalent structure,which is connected to draw vacuum created by vacuum pump 208 from aselected one of first and second material processing chambers 210, 212.Vacuum lines 228, 228A or equivalent structure connect first and secondmaterial processing chambers 210, 212 to vacuum pump 208. A furthervacuum line 230 or equivalent structure connects control valve 206 tovacuum pump 208.

Preferably leading from the bottoms of first and second materialprocessing chambers 210, 212 are a pair of outlet lines 232, 232A orequivalent structure, which in turn connect to first and second driedmaterial flow control valves 218, 220 respectively. First and seconddried material flow control valves 218, 220 or equivalent structurecontrol downward flow of dried granular or powdery resin material fromrespective processing chambers 210, 212 into a reservoir 222 in whichthe dried granular material is retained until needed by themanufacturing process. Line 234 carries material released by valve 218or 220 into reservoir 222. Material feed line 236 carries dried materialas needed from reservoir 222 to a process machine for fabrication wherethe process machine is desirably either a molding press or an extruder.

During operation of vacuum dryer 200, moist granular material requiringdrying is initially fed via valve 204 under the influence of gravityinto the first material processing chamber 210. While in processingchamber 210, the granular resin material is heated, preferably by flowof hot air therethrough, until the material reaches a temperature atwhich vacuum is highly effective to evaporate moisture out of thematerial.

Once application of heat ceases, first material processing chamber isthen sealed so vacuum can be drawn therein and vacuum pump 208 orequivalent structure actuated, with valve 206 or equivalent structureconnecting vacuum pump 208 to first material processing chamber 210.Vacuum is drawn for sufficient time to evaporate the required amount ofmoisture out of the granular resin material within first materialprocessing chamber 210.

While vacuum is being drawn over and moisture is being evaporated fromthe material in processing chamber 210, second material processingchamber 212 has preferably been filled with material and the granularresin material within chamber 212 is heated to the required temperaturefor vacuum-based evaporation of moisture therefrom.

Once the evaporation operation has been completed with respect to thematerial in chamber 210 and the heating has been completed with respectto the material in chamber 212 by virtue of that material having reachedthe required temperature for evaporation of moisture therefrom, theposition of valve 206 may be switched so that vacuum pump 208 draws avacuum within chamber 212 through conduits 228A and 230. During thistime, dried material within chamber 210 may be evacuated via lines 232and 234 by opening valve 218 so that material may flow downwardly intoreservoir 222 and be stored therein until needed for processing by theprocess machine, to which that material may then be carried by line 236.Once first material processing chamber 210 is empty, chamber 210 may berefilled using material from supply 202 by appropriate positioning ofvalve 204 whereupon material may flow from supply 202 via conduits 224,226 into chamber 210 and the process repeated.

Because evaporation of moisture under vacuum is temperature sensitiveand increases greatly in rate with increasing temperature, little isgained by seeking to apply vacuum to the moist granular material beforethe material has been raised to the appropriate temperature. As aresult, a “dual” vacuum dryer system, namely one having two materialprocessing chambers in which one batch of material can be heated while asecond batch of material (having already been heated to the desiredtemperature) is having vacuum drawn thereover and moisture evaporatedtherefrom, is a more efficient system in terms of the amount of driedmaterial delivered per unit time than a system in which vacuum is drawnover the material as the material is being heated.

Vacuum dryer 200 illustrated in FIG. 1 is depicted schematically. Firstand second material processing chambers 210, 212 are desirably equippedwith heated air inlet and outlet hoses, with vacuum inlet and outlethoses and with vacuum sealing means of the type disclosed aboverespecting the invention.

Valve 204 functions as a manifold, preferably being connected to thefirst and second processing chambers 210, 212 and preferably selectablyfurnishes material to be dried to one of the two first and secondprocessing chambers. Desirably, valve 204 acts as a manifold to furnishmaterial to a selected one of first and second chambers 210, 212 mostrecently having dried material evacuated therefrom. Furthermore, it isdesirable that first and second processing chambers 210, 212 haveseparate means for heating material in each of, or associated with,those two chambers.

The apparatus illustrated in FIG. 1 may be modified to utilize only asingle material processing chamber, either 210 or 212, for both heatingand vacuum drying. While this arrangement may be less expensive, it isalso less efficient in that granular material to be dried cannot beeffectively dried under vacuum until heating has been completed, asnoted above.

A second embodiment of a vacuum dryer manifesting aspects of theinvention is illustrated schematically in FIG. 2 with the vacuum dryerbeing designated generally 300 and including a material processingchamber designated generally 302.

A material supply container or equivalent structure is designatedgenerally 304 and serves as a storage receptacle for granular or powderymaterial requiring drying; material supply container 304 need not be apart of dryer 300.

Material processing chamber 302 or equivalent structure is preferablyequipped with a preferably sealing lid designated generally 306 andpositioned to close an inlet end 326 of processing chamber 302 orequivalent structure. Sealing lid 306 is preferably moved by apreferably pneumatic actuating cylinder 308 connected to sealing lid 306by a suitable pivoting arm 310. Upon actuation of cylinder 308, sealinglid 306 moves into position to seal inlet end 326 of processing chamber302.

Granular resin or powdery material requiring drying is conveyed,preferably by gravity, from material supply 304 or equivalent structureto inlet end 326 of processing chamber 302 or equivalent structure viamaterial conveying tube 328.

Material processing chamber 302 is preferably divided into two zones, aheating zone or equivalent structure designated generally 312 and avacuum drying zone or equivalent structure designated generally 314.Zones 312, 314 are preferably separated by a sealing trap door orequivalent structure, such as a slide gate, which is designated 318 andreferred to as a first sealing trap door within preferably cylindricalmaterial processing chamber 302. Heating zone 312 is preferably adaptedto heat granular or powdery material contained therewithin. Anelectrical resistance heater has been designated 316 and is shownschematically as a part of heating zone 312 to indicate the heatingfunction; heating may also be and preferably is provided by hot air inthe manner described above.

A vacuum pump 322 or equivalent structure is preferably connected tovacuum drying zone 314 of processing chamber 302.

The lower or exit end of vacuum drying zone 314 is preferably bounded byand defined by a second sealing trap door or equivalent structuredesignated generally 320 in the drawings. Second sealing trap door 320preferably leads to a dried material discharge conduit 332 providingdried granular or powdery material to a reservoir 324 from whichmaterial may be supplied to a molding machine or extruder as required,preferably via outlet conduit 334.

During operation of the embodiment of the dryer apparatus illustrated inFIG. 2, a first portion of granular or powdery material to be dried ispreferably advanced from a supply in material supply container 304preferably through material inlet conveying tube 328 into heating zone312 of material processing chamber 302. Once within heating zone 312,that first portion of material is heated, preferably by forcing ordrawing hot air through the material. Temperature of the material ispreferably regulated substantially in the same manner as describedabove, namely by comparing temperature of the air going into thematerial and temperature of the air coming out of the material; whenthose air temperatures are equal, the material is known to besubstantially heated to the required temperature.

Once the first portion of heated material is known to be substantiallyat the required temperature, that first portion of material preferablyis advanced from heating zone 312 preferably into vacuum drying zone 314preferably by opening sealing trap door 318 or equivalent structure,such as a slide gate, separating heating zone 312 from vacuum zone 314and allowing the heated material to fall due to gravity from heatingzone 312 into vacuum drying zone 314.

Once the first portion of heated material has been evacuated fromheating zone 312 into vacuum drying zone 314, a second portion of heatedmaterial preferably may be advanced from supply 304 via tube 328 intoheating zone 312, whereupon heating of that batch of material maycommence.

For the first portion of material which is now in vacuum drying zone314, vacuum is preferably drawn over that first portion of material todry the first portion of material while a second portion of material,which is now preferably in heating zone 312, is preferably heated.

Once drying of the first portion of material is substantially completedin vacuum drying zone 314, second sealing trap door 320 or equivalentstructure preferably may be opened and the first portion of material,which is now dried to the required level, may preferably advancedownwardly, preferably due to the force of gravity, through driedmaterial discharge conduit 332, or equivalent structure, into reservoir324 or equivalent structure in which the dried granular material ispreferably stored until needed by the process machine.

These steps of advancing portions of granular material from the supplyinto the heating zone, heating of material in the heating zone while thenext preceding portion of material is being dried in the vacuum dryingzone, and then advancing the two portions of material successively fromthe drying zone into the reservoir and from the heating zone into thedrying zone, may preferably be repeated until such time as no additionaldried material is required by the process machine to which conduit 334is connected or leads.

Referring to FIG. 3, an air purge granular resin material vacuum dryerin accordance with the invention is shown schematically and designatedgenerally 10. Air purge dryer 10 includes a heating hopper 12, a vacuumchamber 14, and a retention hopper 16, with the heating hopper beingpositioned above the vacuum chamber and the vacuum chamber in turn beingpositioned above the retention hopper 16, with the heating hopper 12,vacuum chamber 14, and retention hopper 16 being desirably verticallysubstantially aligned, as shown in FIG. 3.

Heating hopper 12, vacuum chamber 14, and retention hopper 16 are allpreferably independently supported by a support frame designated 20shown only schematically in FIG. 1. Preferably, heating hopper 12 doesnot rest on vacuum chamber 14. To the contrary, support frame 20preferably supports heating hopper 12 above vacuum chamber 14 so thatnone of the weight of heating hopper 12 or any resin within heatinghopper 12 is supported by vacuum chamber 14. Heating hopper 12 isdesirably an insulated stainless steel hopper and can accommodate dryingtemperatures of up to 350° F. The heating temperature is adjusted or seton a control panel portion of controller 76.

Similarly, vacuum chamber 14 is preferably independently supported bysupport frame 20 so that none of the weight of vacuum chamber 14 istransferred to or borne by retention hopper 16. While support frame 20has been depicted in FIG. 3 in three sections, it is to be understoodthat support frame 20 can be a single structural member so long assupport frame 20 provides separate weight-bearing support for heatinghopper 12, vacuum chamber 14, and retention hopper 16. While supportframe 20 has been illustrated in the drawing as being under vacuumchamber 14, vacuum chamber 14 may also desirably and most preferably besuspended from above by a suitable frame member similar to schematicframe 20.

The vertically aligned “stacked” arrangement of heating hopper 12,vacuum chamber 14, and retention hopper 16, as depicted generally inFIG. 3, permits gravity-induced flow of granular plastic resin fromheating hopper 12 downwardly into vacuum chamber 14, and from vacuumchamber 14 downwardly into retention hopper 16. Desirably, retentionhopper 16 is supported by support frame 20 in a manner that retentionhopper 16 is somewhat above floor level in the facility in which airpurge dryer 10 is located. Having retention hopper 16 above the floorpermits dried granular resin material to be supplied directly out ofretention hopper 16 by gravity flow to a process machine such as amolding press or an extruder, or to a vacuum-powered or pneumaticallypowered resin distribution system within the processing facility.Support frame 20 has been illustrated in schematic form as supportingretention hopper 16, vacuum chamber 14 and heating hopper 12; desirablyin addition to vacuum chamber 14 being mountable on rails and in asuspended disposition from support frame 20, heating hopper 12 andretention hopper 16 may both also be mounted on rails to facilitatemovement and removal of heating hopper 12 and retention hopper 16 asneeded for maintenance, etc.

Air for heating granular plastic resin within heating hopper 12 ispreferably supplied by a centrifugal blower 22 that draws in ambient airand forces that ambient air through an air heating chamber 23, whichpreferably includes a heating element 24 positioned within an open endedcylindrical housing 25. The open ended cylindrical housing 25 ispreferably a 6 inch diameter, 6 inch length stainless steel cylinderhaving suitable insulative material around the exterior thereof. Voltageapplied to heating element 24 within cylindrical housing 25 causesheating element 24 to rise in temperature. Air passing along heatingelement 24, as blown through air heating chamber 23 by centrifugalblower 22, is heated by heating element 24 and exits air heating chamber23 at the top of chamber 23 and travels via a hot air conduit 74 toheating hopper 12, where the hot air enters heating hopper 12 at thebottom thereof for upward passage through granular plastic resinmaterial residing in heating hopper 12. A variable frequency drive 30 isprovided for centrifugal blower 22 to modulate the speed of blower 22and thereby control and adjust the amount of heating air, and thereforethe amount of heat introduced into heating hopper 12.

Vacuum chamber 14 is mounted on support frame 20 with one or more loadcells 36 between vacuum chamber 14 and support frame 20. Load cell 36provides data to controller 76 as to the weight of vacuum chamber 20 andany granular plastic resin material being dried therein.

Similarly, retention hopper 16 is mounted on support frame 20 using oneor more load cells 38 to provide data to controller 76 as to the weightof dried granular plastic resin material resident within retentionhopper 38.

Temperature sensors are provided to monitor air temperature at the inletconnecting conduit 74 to heating hopper 12 and at the top of heatinghopper 12, where the heated air, having given up most of its heat, isexhausted. The temperature sensor at the hot air inlet to heating hopper12 is designated 44 in the drawings, while the temperature sensor at theoutlet, at the top of heating hopper 12 where heated ambient air isexhausted, is designated 46.

A material level sensor 42 is provided in heating hopper 12. Levelsensor 42 provides a signal indicating excessively low level of materialin heating hopper 12. Controller 76 receives a signal from heatinghopper level sensor 42 and in response to a low material level signal,controller 76 either actuates apparatus to provide granular resinmaterial for replenishing heating hopper 12 or, if no material isavailable, controller 76 shuts down the air purge dryer 10.

A temperature sensor 56 within retention hopper 16 senses thetemperature of the dry purge air with which dried granular resin inretention hopper 16 is blanketed. A granular resin material temperaturesensor 58 may be provided at the bottom, close to the material outletfrom retention hopper 16, to sense the temperature of the resin materialbeing supplied from retention hopper 16.

Controller 76 desirably has two display screens. The upper screen 82,which desirably has a red background, shows actual temperatures and setpoint temperatures. The lower screen 84, which desirably has a bluebackground, shows various running mode information, set up information,and dryer configuration information, as selected by the operator usingtouch controls that are a part of controller 76 and are associated withthe two screens.

One or more oil separator coalescing filters 32 are provided to removeentrained oil and some moisture from the compressed air supply. Acompressed air membrane dryer 34 further dries the air and provides verydry purge air for vacuum chamber 14 and a dry air blanket formaintenance of dry conditions for granular resin material in retentionhopper 16.

As operation of the air purge dryer begins, material in heating hopper12 is brought up to temperature. The time for preheating is determinedby a specified preheat time, which may be entered by an operator intocontroller 76, or by an automatic set-up option in controller 76 whichestablishes an inlet-to-outlet temperature difference for the air inputto and exhausted by heating hopper 12, and a minimum preheat time. Onceresin material in heating hopper 12 is up to temperature, as determinedby the inlet-to-outlet temperature difference as measured by temperaturesensors 44 and 46, and the temperature difference is supplied tocontroller 76, approximately one-third of the resin material in heatinghopper 12 is dispensed into vacuum chamber 14. Once this occurs, a firstvacuum drying cycle begins. Each vacuum drying cycle, namely the time abatch of resin material remains in vacuum chamber 14 under vacuum, has aminimum time that the material is maintained under vacuum. This time maybe set by an operator using the inputs available on controller 76 or adefault time of twenty (20) minutes may be used.

During normal operation, vacuum in vacuum chamber 14 is brought to alevel of about 700 mm Hg and held to about a plus or minus 20 mm Hgdifferential for the vacuum cycle time. A typical vacuum cycle lastsfrom 15 to 20 minutes, depending on the material being dried.

As vacuum chamber 14 receives the heated granular resin material throughfirst conduit 102 via operation of material flow control gates 60 and 62and the vacuum cycle begins, a suitable loader, either human ormechanical, such as the loader that is the subject of U.S. Pat. No.8,753,432, loads heating hopper 12 with new replenishment material,desirably concurrently with the start of the vacuum cycle. Granularresin material loaded into heating hopper 12 remains in heating hopper12 for a minimum of the time for a vacuum cycle in vacuum chamber 14.

After completion of a vacuum cycle in vacuum chamber 14, granular resinmaterial that has been dried in vacuum chamber 14 is dispenseddownwardly through second conduit 104, via operation of material flowcontrol gates 64 and 66, into retention hopper 16 and is ready for use.Dried granular resin material residing in retention hopper 16 and notimmediately removed therefrom for molding or extrusion is blanketed withdry air so long as that granular resin material remains in retentionhopper 16. The dry air blanketing the dried granular resin materialremaining in retention hopper 16 is maintained under positive pressureand is desirably slightly heated so as to be warm. The dry airblanketing the dried granular resin material remaining in retentionhopper 16 prevents that granular resin material from absorbing moisture,which would render the material unsuitable for subsequent processing bymolding or extrusion.

The rate of consumption of dried granular resin material from retentionhopper 16 dictates the time granular resin material will be heated inheating hopper 12 and dried under vacuum in vacuum chamber 14. Forexample, if thirty (30) minutes are required for operation of a processmachine to deplete retention hopper 16, the vacuum cycle in vacuumchamber 14 will run past the normal twenty (20) minute set point andwill last thirty (30) minutes. This is normal operation and does not inany way degrade the granular plastic resin that has been dried in vacuumchamber 14. However, if retention hopper 16 is depleted in fifteen (15)minutes due to being consumed by operation of a process machine and thetime for a vacuum cycle in vacuum chamber 14 has been set to twenty (20)minutes, a five (5) minute window will result when no granular resinmaterial is available for the process machine. This indicates that thethroughput capacity of the dryer has been exceeded for the particulargranular resin material being dried and the particular process machinebeing supplied. Upon such occurrence, controller 76 senses thatretention hopper 16 is empty, that vacuum chamber 14 is still dryingmaterial, and with no material being available in retention hopper 16for the associated process machine, controller 76 sounds an alarm.

Vacuum chamber load cell(s) 36 and retention hopper load cell(s) 38allow controller 76 to always have in memory the current weight ofmaterial in the vacuum chamber and the current weight of material in theretention hopper. This permits calculation by controller 76 ofthroughput of granular resin material in pounds of resin material perhour.

Venturi vacuum generator 28 requires an operating air pressure of about80 psi. The pressurized air is desirably supplied by an in-house airsystem.

A purge air inlet temperature sensor 56 is provided in retention hopper16. A granular resin material outlet temperature sensor 58 is providedat the bottom of retention hopper 16. Both sensor 56 and sensor 58provide temperature data to controller 76.

The desired temperature of air being outlet from the top of heatinghopper 12 may be set in controller 76 such that once the temperature ofair escaping from the top of heating hopper 12 reaches a desired level,centrifugal blower 22 and heating element 24 will shut down for apredetermined time period specified by an operator and programmed intocontroller 76 or until a vacuum cycle, which is under way, ends,whichever event comes first.

Fill and the fill rate for vacuum chamber 14 are controlled and may beadjusted by material flow control gates 60 and 62 above vacuum chamber14 as actuated and controlled by controller 76. Similarly, material dumpand material dump rate from vacuum chamber 14 can be controlled andadjusted by material flow control gates 64 and 66 below vacuum chamber14 as actuated and controlled by controller 76. These parameters, namelyvacuum chamber fill and fill rate and vacuum chamber dump and dump rateare programmable into controller 76. Similarly, the timing by which drypurge air is introduced into vacuum chamber 14 is desirably adjusted andcontrolled by controller 76. Typically during a twenty (20) minutevacuum cycle, purge air will be introduced into vacuum chamber 14 six(6) times.

Controller 76 controls and allows adjustment of the heat provided toheating hopper 12.

While the vacuum dryer of the invention produces dried material inbatches, the dryer is a continuous supplier of suitably dry material formolding or extrusion. Dry material may be withdrawn from retentionhopper 16 on a continuous basis. Vacuum chamber 14 preferably processesone batch of material every 20 minutes, which is sufficient to keepretention hopper 16 and any process machine being fed by retentionhopper 16 supplied on a continuous basis.

The vacuum dryer of the invention uses fresh air without recycling anyair in the dryer. Air coming into the dryer is used once and goes out ofthe dryer; there is no recycling of air in this embodiment of theinvention.

The load cells, together with controller 76, facilitate trackingthroughput of granular resin material by the vacuum dryer of theinvention, permitting optimization of manufacturing parameters in theplastic molding or extrusion facility in which the dryer of theinvention is located.

During operation, vacuum is drawn by Venturi vacuum generator 28 fromvacuum chamber 14 via vacuum drawing conduit 90.

Incoming compressed air from the plastics molding or extrusion facilityis supplied to pressure regulator 100 as indicated FIG. 3. Thisregulated pressurized air, with pressure regulated to a required level,is then supplied via regulated pressure air line 106, which splits asillustrated in FIG. 3 with one portion of line 106 leading to oilseparating coalescing filter 32 and the other portion of line 106leading to Venturi vacuum generator 28. An exhaust line 92 leads fromVenturi vacuum generator 28 to ambient air.

Purge air is provided via purge air supply line 94 which exitscompressed air membrane dryer 34 and supplies purge air in very dry formafter exiting dryer 34 to both retention hopper 16 and to vacuum chamber14. Introduction of purge air to retention hopper 16 is controlled byvalve 96, which in turn is actuated by controller 76. Introduction ofpurge air to vacuum chamber 14 is controlled by vacuum chamber purge airvalve 98, which in turn is also controlled by controller 76. The wiringfor connection of valves 96, 98 and the other components to controller76 is not illustrated in the drawing to enhance the drawing clarity.

Flow of granular plastic resin material downwardly from heating hopper12 to vacuum chamber 14 is desirably through a first conduit 102. Flowof dried granular resin material from vacuum chamber 14 to retentionhopper 16 is desirably through a second conduit 104. Conduits 102, 104are respectively mechanically connected, preferably substantially airtightly, respectively to heating hopper 12, vacuum chamber 12 andretention hopper 16.

Gates 60, 62, 64, and 66 have been illustrated in FIG. 3 positionedrespectively in the bottom of heating hopper 60, at the top and at thebottom of vacuum chamber 14, and at the top of retention hopper 16.These gates may desirably be positioned in respective first and secondconduits 102, 104 according to the manner of selected construction forthe flow through vacuum dryer.

It is desirable to have two gates, such as gates 60, 62, above vacuumchamber 14 to control downward flow of resin from heating hopper 12,with an upper gate 60 providing gross, course control and a lower gate62 providing air tight vacuum sealing of the vacuum chamber. Use of thetwo gates, 60, 62, with course control afforded by upper gate 60,minimizes the possibility of resin material becoming stuck in gate 62and thereby precluding gate 62 from making the vacuum tight sealrequired for effective operation of vacuum chamber 14 during the dryingphase. Desirably, gate 62 is a slide gate providing a vacuum tight sealusing a rubber gasket, with the movable slide portion of the gateclosing against the rubber gasket and moving first in a directionlaterally across, with respect to the direction of downward flow ofresin, and then vertically parallel with the direction of downward flowof resin, with such horizontal and then vertical movement of the gateeffectuated by the shape of the slot in which the slide gate moves.

Material gate 64 may similarly be a slide gate or may be a pivotinggasket-equipped gate actuated by an air cylinder with the gate pivotingdownwardly to effectuate downward flow of dried plastic resin materialout of vacuum chamber 14 upon the conclusion of the vacuum cycle. Use ofa pivoting-type gate at gate 64 reduces cost over the cost of a slidegate since gravity will carry any residual granules of plastic resinmaterial downwardly through second conduit 104 into retention hopper 16.Gates 60 and 66 may be of any suitable type, desirably actuated by aircylinders controlled by controller 76.

All components illustrated in FIG. 3 are controlled by controller 76.This includes the drive 30 for centrifugal blower 22, heating element23, the various gates that control the flow of resin downwardly throughthe dryer, the load cells that detect weight thereby allowing thecomputation of amount of material flowthrough, and the like. Controller76 controls all aspects of the operation of the dryer and once the dryeris started, human intervention is not necessary. Of course, controlsprovided on controller 76 to allow human intervention if desired.

Conventional industry practice is to dry, then blend, and then processgranular resin material using a desiccant dryer, then a gravimetricblender and then a molding machine. The dryer of the inventionfacilitates reversal of the first two stages of that process, namelypermitting drying to be done after measuring and blending. This isadvantageous because of problems associated with desiccant dryersincluding separation of the blend resulting in a large quantity of resinmaterial being already preblended that might not be usable in the eventof such separation. This is the reason desiccant dryers areconventionally used prior to or upstream of gravimetric blenders in theplastics molding industry. Since the invention facilitates drying ofgranular material after the measuring and blending of such material, theinvention eliminates risk involved in storing preblended material,namely separation of the blend which may render the material unusable.

With dryers of the invention, removal of moisture is on the order of2/10 of 1% of the weight of the material so there is no adverse effecton the blend or on the proportions of the blend that have beeneffectuated by a gravimetric blender positioned upstream of a dryer inaccordance with the invention.

Dryers in accordance with the invention uniformly and consistentlyexhibit a six-fold reduction in drying time over that experienced usingconventional desiccant dryers when drying granular plastic resinmaterial prior to molding or extrusion. Such conventional desiccantdryers rely entirely on blowing warm air over the plastic material andhaving the warm, dried air absorb moisture out of the plastic materialof interest.

In the dryers according to the invention, especially the embodimentsshown in FIGS. 1 and 2, the vacuum drawn during the drying process canbe as low from one to three inches of mercury short of absolute vacuum.Hence, under standard conditions these dryers preferably develop avacuum of from 27 to 29 inches of mercury in the vacuum drying canister.

Preferably a dryer supplies hot air to heat granular resin material atfill and heat position 100 at a temperature as high as 260° F. or evenas high as 300° F.

In a typical application where a molding machine may require 100 poundsper hour of processed, dried, ready to mold plastic resin, a dryer inaccordance with the invention can supply about 105 pounds of materialper hour, ready to be processed by the molding machine.

For a molding machine operating with a conventional desiccant dryersupplying granular resin material at the same 100 pounds of materialthroughput per hour, a desiccant dryer having capacity of 400 poundswould be required in order to provide the 100 pounds per hour ofmaterial, due to the four hour desiccant drying time.

Dryers in accordance with the invention take up less space and generallyprovide a more efficient operation for a molder than a conventionaldesiccant dryer.

A desiccant dryer process requiring 100 pounds throughput of materialper hour requires a four hour lead time since such a desiccant dryertypically requires four hours to provide the first batch of material atacceptable dryness. In contrast a dryer in accordance with the inventiononly needs 40 minutes or less to provide the first batch of material atacceptable dryness for startup of the molding operation.

A new color may be introduced into the drying procedure while thepreceding color or final batch of plastic resin material with thepreceding colors is being dried and delivered. Hence there is nointerruption in operation of the dryers of the invention in order tochange colors of the granular resin material being dried. In contrast, aconventional desiccant dryer would require four hours of down time inorder to change the color of the granular plastic resin being dried.

Dryers in accordance with the invention make economical the recycling ofnylon scrappage which heretofore has been practical due to the dryingtime required for such scrappage. When nylon is conventionally processedand scrap nylon results as a byproduct of the process, in some cases itmay take up to three days, using known methods and equipment, to dry thescrap nylon to a sufficient extent that the nylon can be reground andreprocessed. A dryer in accordance with the invention has been testedexperimentally on such nylon recyclage and has been found to adequatelyprocess the nylon recyclage in six hours, amounting to a 92% reductionin drying time over that known heretofore. Hence, use of a dryer inaccordance with the invention may provide a source of continuous supplyof dried reprocessable nylon for recycling which has heretofore not beenpractical due to the affinity of nylon for moisture and the length oftime it has taken to dry nylon recyclage to a sufficient degree to makeit processable in a recycling mode.

Another important advantage of the invention is that plastic resinmaterial being dried is exposed to heat for a much shorter time thanwith known methods, thereby reducing the risk of plastic degradation dueto exposure to heat. Many molding materials, especially more expensivemolding materials, are highly sensitive to exposure to heat. Thesematerials, commonly referred to as “engineering” materials, includenylon, PET and various polycarbonates.

The foregoing describes the preferred embodiment and alternateembodiments of the invention and sets forth the best mode contemplatedfor carrying out the invention in such terms as to facilitate practiceof the invention by a person of ordinary skill in the art. However, itis to be understood that the invention has many aspects, is not limitedto the structure, processes, methods and embodiments disclosed and/orclaimed, and that equivalents to the disclosed structure, processes,methods, embodiments and claims are within the scope of the invention asdefined by the claims appended hereto or added subsequently.

In the claims, “comprising” means “including, but not limited to”, while“consisting of” means “having and no more”, with both definitions beingin accordance with conventional patent application prosecution procedurein the United States Patent and Trademark Office.

What is claimed is:
 1. A method for drying granular plastic resin priorto molding plastic into finished products comprising sequentiallyheating and vacuum drying batch portions of plastic resin material witha next succeeding batch portion being heated while a current batchportion is being vacuum dried.
 2. The method of claim 1 furthercomprising performing the heating and vacuum drying to supply driedplastic resin material at the rate of material consumption by anassociated process machine.
 3. Apparatus for drying granular resinmaterial prior to molding or extrusion processing thereof, comprising:a. a heating chamber; b. a vacuum chamber below the heating chamber; c.a retention hopper below the vacuum chamber; d. a blower for supplyingheated ambient air upwardly through the heating chamber; e. a conduitfor introducing dry purge air into the vacuum chamber; and f. a conduitfor introducing blanketing dry air into the retention hopper. 4.Apparatus of claim 3 further comprising: a. a first support bearing theweight of the heating hopper; and b. a second support bearing the weightof the vacuum chamber.
 5. Apparatus of claim 4 wherein the vacuumchamber is suspended from the second support.
 6. Apparatus of claim 4wherein the first and second supports are connected.
 7. Apparatus ofclaim 3 further comprising: a. a heater for heating ambient air forintroduction into the heating hopper, comprising: i. a hollow housing;and ii. an electrical heating element within the housing.
 8. Apparatusof claim 3 further comprising: a. a first gate between the heatingchamber and the vacuum chamber, movable between an open and closedpositions, for controlling downward resin flow into the vacuum chamber;b. a second gate between the vacuum chamber and the retention hopper,movable between open and closed positions, for controlling downwardresin flow from the vacuum chamber into the retention hopper. 9.Apparatus of claim 8 further comprising: a. a plurality of first gatesbetween the heating chamber and the vacuum chamber, movable between anopen and closed positions, for controlling downward resin flow into thevacuum chamber; b. a plurality of second gates between the vacuumchamber and the retention hopper, movable between open and closedpositions, for controlling downward resin flow into the retentionhopper.
 10. Apparatus of claim 9 wherein the plurality of first gatesincludes at least one slide gate.
 11. Apparatus of claim 9 wherein afirst one of each of the two pluralities of gates are connected to thevacuum chamber.
 12. Apparatus of claim 8 wherein the first gate is aslide gate.
 13. Apparatus of claim 3 further comprising a sensor forsensing weight of the vacuum chamber hopper and any resin materialtherein.
 14. Apparatus of claim 9 further comprising a sensor forsensing weight of the retention hopper and any resin material therein.15. Apparatus of claim 4 further comprising a frame connected to thefirst and second supports.
 16. Apparatus of claim 3 wherein the sensorsare load cells.
 17. Apparatus of claim 3 further comprising: a. anadjustable blower for blowing warm air through the heater and into theheating chamber; b. a sensor for detecting air temperature at the top ofthe heating hopper; and c. a control for adjusting speed of the blowerin response to the detected air temperature.
 18. A method for dryinggranular resin material prior to processing thereof by molding orextrusion, comprising: a. heating granular resin material in a heatinghopper; b. monitoring air temperature at the top of the heating hopper;c. regulating heat input to the heating hopper so that monitored airtemperature at the top of the heating hopper dies not exceed apreselected temperature; d. releasing heated granular resin materialfrom the heating hopper for flow downwardly into a vacuum chamber whilereplenishing the heating hopper from above with fresh resin material inan amount substantially equal to that released into the vacuum chamber;e. drawing vacuum in the vacuum chamber while periodically purging thevacuum chamber with dry air; f. draining resin material from the vacuumchamber into a retention hopper; g. blanketing resin material in theretention chamber with dry air.
 19. The method of claim 18 whereinregulating heat input to the heating hopper further comprises regulatingspeed of air passing a heating element.
 20. The method of claim 18wherein heating the granular resin further comprises introducing heatedambient air into the hopper.
 21. The method of claim 20 wherein heatinggranular resin material in the heating hopper further comprisesintroducing heat into the hopper at the hopper bottom.
 22. The method ofclaim 18 wherein purging the vacuum chamber with dry air comprises: a.passing compressed air along a membrane dryer; b. introducing thecompressed air into the vacuum chamber at the bottom of the chamber. 23.In a method for drying granular resin material prior to processingthereof by molding or extrusion, the improvement comprising: a.introducing heated air into a granular resin material storage hopper atthe hopper bottom; and b. monitoring the temperature of air leaving thehopper at a position above granular resin material in the hopper andregulating the rate of heated air introduction into the hopper so thatmonitored temperature of air leaving the hopper does not exceed apreselected level.
 24. The method of claim 23 further comprising: a.drawing vacuum in the hopper thereby drawing moisture from the granularresin material; and b. draining granular resin material from the hopperas needed for processing.